Date of Award


Degree Type


Degree Name

Doctor of Philosophy (PhD)


Physics and Astronomy

First Advisor

H. Richard Miller - Chair

Second Advisor

D. Michael Crenshaw

Third Advisor

Todd J. Henry

Fourth Advisor

William H. Nelson

Fifth Advisor

Paul J. Wiita


Strong and variable X-ray emission has long been known to be a universal property of active galaxies. However, despite years of study, the exact nature of the variability remains relatively unknown. We present here results of a multi-year monitoring campaign of a sample of Seyfert galaxies (3C 120, Mkn 509, 3C 390.3, and Akn 120), carried out using the Rossi X-ray Timing Explorer (RXTE). For Mkn 509, we also present results of optical monitoring. Mkn 509 shows a strong correlation between X-ray and optical variations, with the optical leading the X-ray by 25 days. We also investigate the rms-flux relationship in our sample. The two radio loud objects in our sample (3C 120, 3C 390.3) show a clear correlation between flux and rms variability, while the two radio quiet objects (Mkn 509, Akn 120) show no such relationship. Monte Carlo simulations were used to estimate the shape of the underlying power spectrum, and we find that all of our objects have a break frequency below which the power spectrum flattens. The relationship between optical and X-ray variability is discussed, with lags occurring most likely as a result of instabilities or changes in accretion flow propagating inwards through the disk. We also discuss possible physical timescales that could be related to the break frequency, along with connections to galactic X-ray binaries.